I have the same issue, So I connected USB B directly to Topping E30 II Dac to get AKM signature sound and then connected to line in of WiiM Amp to get PEQ, Room correction and Subwoofer out. You may be critical to connect so, but I found no other choice to get all above mentioned features I like in Wiim ecosphere.
BTW once the USB out with parallel subwoofer option available as promised by WiiM developer team, I will run by the above method.
I am very please with the DAC analogue out then redigitalizing for the facilities. Someone who is knowledgeable how much quality degradation may happen in this.
Keep in mind that I found no other way to get UAPP to play in actual footprint (Hz, bit rate) and sending the best available signals to WiiM..